Enhanced magnetic susceptibility in Ti3C2Tx MXene with Co and Ni incorporation

Magnetic nanomaterials are sought to provide new functionalities for applications ranging from information processing and storage to energy generation and biomedical imaging. MXenes are a rapidly growing family of two-dimensional transition metal carbides and nitrides with versatile chemical and structural diversity, resulting in a variety of interesting electronic and optical properties. However, strategies for producing MXenes with tailored magnetic responses remain underdeveloped and challenging. Herein, we incorporate elemental Ni and Co into Ti3C2Tx MXene by mixing with dilute metal chloride solutions. We achieve a uniform distribution of Ni and Co, confirmed by X-ray fluorescence (XRF) mapping with nanometer resolution, with Ni and Co concentrations of approximately 2 and 7 at% relative to the Ti concentration. The magnetic susceptibility of these Ni- and Co-incorporated Ti3C2Tx MXenes is one to two orders of magnitude larger than pristine Ti3C2Tx, illustrating the potential for dilute metal incorporation to enhance linear magnetic responses at room temperature.

Pristine Ti3C2Tx MXene Enables Flexible and Transparent Electrochemical Sensors

In the era of the internet of things, there exists a pressing need for technologies that meet the stringent demands of wearable, self-powered, and seamlessly integrated devices. Current approaches to developing MXene-based electrochemical sensors involve either rigid or opaque components, limiting their use in niche applications. This study investigates the potential of pristine Ti3C2Tx electrodes for flexible and transparent electrochemical sensing, achieved through an exploration of how material characteristics (flake size, flake orientation, film geometry, and uniformity) impact the electrochemical activity of the outer sphere redox probe ruthenium hexamine using cyclic voltammetry. The optimized electrode made of stacked large Ti3C2Tx flakes demonstrated excellent reproducibility and resistance to bending conditions, suggesting their use for reliable, robust, and flexible sensors. Reducing electrode thickness resulted in an amplified faradaic-to-capacitance signal, which is advantageous for this application. This led to the deposition of transparent thin Ti3C2Tx films, which maintained their best performance up to 73% transparency. These findings underscore its promise for high-performance, tailored sensors, marking a significant stride in advancing MXene utilization in next-generation electrochemical sensing technologies. The results encourage the analytical electrochemistry field to take advantage of the unique properties that pristine Ti3C2Tx electrodes can provide in sensing through more parametric studies.

Effect of the deposition process on the stability of Ti3C2Tx MXene films for bioelectronics

Ti₃C₂T_x MXene is emerging as the enabling material in a broad range of wearable and implantable medical technologies, thanks to its outstanding electrical, electrochemical, and optoelectronic properties, and its compatibility with high-throughput solution-based processing. While the prevalence of Ti₃C₂T_x MXene in biomedical research, and in particular bioelectronics, has steadily increased, the long-term stability and degradation of Ti₃C₂T_x MXene films have not yet been thoroughly investigated, limiting its use for chronic applications. Here, we investigate the stability of Ti₃C₂T_x films and electrodes under environmental conditions that are relevant to medical and bioelectronic technologies: storage in ambient atmosphere (shelf-life), submersion in saline (akin to the in vivo environment), and storage in a desiccator (low-humidity). Furthermore, to evaluate the effect of the MXene deposition method and thickness on the film stability in the different conditions, we compare thin (25 nm), and thick (1.0 μm) films and electrodes fabricated via spray-coating and blade-coating. Our findings indicate that film processing method and thickness play a significant role in determining the long-term performance of Ti₃C₂T_x films and electrodes, with highly aligned, thick films from blade coating remarkably retaining their conductivity, electrochemical impedance, and morphological integrity even after 30 days in saline. Our extensive spectroscopic analysis reveals that the degradation of Ti₃C₂T_x films in high-humidity environments is primarily driven by moisture intercalation, ingress, and film delamination, with evidence of only minimal to moderate oxidation.

M5X4: A Family of MXenes

MXenes are two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides typically synthesized from layered MAX-phase precursors. With over 50 experimentally reported MXenes and a near-infinite number of possible chemistries, MXenes make up the fastest-growing family of 2D materials. They offer a wide range of properties, which can be altered by their chemistry (M, X) and the number of metal layers in the structure, ranging from two in M2XTx to five in M5X4Tx. Only one M5X4 MXene, Mo4VC4, has been reported. Herein, we report the synthesis and characterization of two M5AX4 mixed transition metal MAX phases, Ti2.5Ta2.5AlC4 and Ti2.675Nb2.325AlC4, and their successful topochemical transformation into Ti2.5Ta2.5C4Tx and Ti2.675Nb2.325C4Tx MXenes. The resulting MXenes were delaminated into single-layer flakes, analyzed structurally, and characterized for their thermal and optical properties. This establishes a family of M5AX4 MAX phases and their corresponding MXenes. These materials were experimentally produced based on guidance from theoretical predictions, leading to more exciting applications for MXenes.

All-Liquid Reconfigurable Electronics Using Jammed MXene Interfaces

Rigid, solid-state components represent the current paradigm for electronic systems, but they lack post-production reconfigurability and pose ever-increasing challenges to efficient end-of-life recycling. Liquid electronics may overcome these limitations by offering flexible in-the-field redesign and separation at end-of-life via simple liquid phase chemistries. Up to now, preliminary work on liquid electronics has focused on liquid metal components, but these devices still require an encapsulating polymer and typically use alloys of rare elements like indium. Here, using the self-assembly of jammed 2D titanium carbide (Ti₃ C₂ T_x ) MXene nanoparticles at liquid-liquid interfaces, "all-liquid" electrically conductive sheets, wires, and simple functional devices are described including electromechanical switches and photodetectors. These assemblies combine the high conductivity of MXene nanosheets with the controllable form and reconfigurability of structured liquids. Such configurations can have applications not only in electronics, but also in catalysis and microfluidics, especially in systems where the product and substrate have affinity for solvents of differing polarity.

Ultralarge Flakes of Ti3C2Tx MXene via Soft Delamination

Two-dimensional (2D) titanium carbide MXene (Ti₃C₂T_x) has attracted significant attention due to its combination of properties and great promise for various applications. The size of the 2D sheets is a critical parameter affecting multiple properties of assembled films, fibers and 3D structures. The increased lateral size of MXene flakes can benefit not only their assemblies by improving the interflake contacts and alignment but also fundamental studies at the individual flake level, allowing for facile patterning and investigation of intrinsic physical properties of MXenes. Increasing the average size of the parent MAX phase is one of the strategies previously used to increase the flake size of the resultant MXene. Here, we show that the protocol used for the next step of the synthesis procedure, delamination of multilayer MXene into individual nanosheets, significantly affects the lateral size of the resultant flakes. We developed a soft delamination approach, which prevents fracture of flakes and preserves their size. Combining this approach with the large-grain Ti₃AlC₂ MAX phase precursor, we achieved individual flakes of up to 40 μm in lateral size. These flakes can be used for patterning multiple contacts and fabrication of field-effect transistors for multiprobe electrical characterization and other measurements. These findings indicate the importance of controlling the delamination process in order to achieve large MXene flakes and improve properties of MXene-based materials and devices.

Highly Selective Gas Sensors Based on Graphene Nanoribbons Grown by Chemical Vapor Deposition

Despite the recent advances in bottom-up synthesis of different kinds of atomically precise graphene nanoribbons (GNRs) with very diverse physical properties, the translation of these GNRs into electronic devices remains challenging. Among other factors, the electronic characterization of GNRs is hampered by their complex synthesis that often requires custom-made organic precursors and the need for their transfer to dielectric substrates compatible with the conventional device fabrication procedures. In this paper, we demonstrate that uniform electrically conductive GNR films can be grown on arbitrary high-temperature-resistant substrates, such as metals, Si/SiO₂, or silica glasses, by a simple chemical vapor deposition (CVD) approach based on thermal decomposition of commercially available perylenetetracarboxylic dianhydride molecules. The results of spectroscopic and microscopic characterization of the CVD-grown films were consistent with the formation of oxygen-terminated N = 5 armchair GNRs. The CVD-grown nanoribbon films exhibited an ambipolar electric field effect and low on-off ratios, which were in agreement with the predicted metallic properties of N = 5 armchair GNRs, and remarkable gas sensing properties to a variety of volatile organic compounds (VOCs). We fabricated a GNR-based electronic nose system that could reliably recognize VOCs from different chemical classes including alcohols (methanol, ethanol, and isopropanol) and amines (n-butylamine, diethylamine, and triethylamine). The simplicity of the described CVD approach and its compatibility with the conventional device fabrication procedures, as well as the demonstrated sensitivity of the GNR devices to a variety of VOCs, warrant further investigation of CVD-grown nanoribbons for sensing applications.

Phenyl Functionalization of Atomically Precise Graphene Nanoribbons for Engineering Inter-ribbon Interactions and Graphene Nanopores

Graphene nanoribbons (GNRs) attract much attention from researchers due to their tunable physical properties and potential for becoming nanoscale building blocks of electronic devices. GNRs can be synthesized with atomic precision by on-surface approaches from specially designed molecular precursors. While a considerable number of ribbons with very diverse structures and properties have been demonstrated in recent years, there have been only limited examples of on-surface synthesized GNRs modified with functional groups. In this study, we designed a nanoribbon, in which the chevron GNR backbone is decorated with phenyl functionalities, and demonstrate the on-surface synthesis of these GNRs on Au(111). We show that the phenyl modification affects the assembly of the GNR polymer precursors through π-π interactions. Scanning tunneling spectroscopy of the modified GNRs on Au(111) revealed that they have a band gap of 2.50 ± 0.02 eV, which is comparable to that of the parent chevron GNR. The phenyl functionalization leads to a shift of the band edges to lower energies, suggesting that it could be a useful tool for the GNR band structure engineering. We also investigated lateral fusion of the phenyl-modified GNRs and demonstrate that it could be used to engineer different kinds of atomically precise graphene nanopores. A similar functionalization approach could be potentially applied to other GNRs to affect their on-surface assembly, modify their electronic properties, and realize graphene nanopores with a variety of structures.

Inkjet printable-photoactive all inorganic perovskite films with long effective photocarrier lifetimes

Photoactive perovskite quantum dot films, deposited via an inkjet printer, have been characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The crystal structure and bonding environment are consistent with CsPbBr₃ perovskite quantum dots. The current-voltage (I-V) and capacitance-voltage (C-V) transport measurements indicate that the photo-carrier drift lifetime can exceed 1 ms for some printed perovskite films. This far exceeds the dark drift carrier lifetime, which is below 50 ns. The printed films show a photocarrier density 10⁹ greater than the dark carrier density, making these printed films ideal candidates for application in photodetectors. The successful printing of photoactive-perovskite quantum dot films of CsPbBr₃, indicates that the rapid prototyping of various perovskite inks and multilayers is realizable.

Dense monolayer films of atomically precise graphene nanoribbons on metallic substrates enabled by direct contact transfer of molecular precursors

Atomically precise graphene nanoribbons (GNRs) of two types, chevron GNRs and N = 7 straight armchair GNRs (7-AGNRs), have been synthesized through a direct contact transfer (DCT) of molecular precursors on Au(111) and gradual annealing. This method provides an alternative to the conventional approach for the deposition of molecules on surfaces by sublimation and simplifies preparation of dense monolayer films of GNRs. The DCT method allows deposition of molecules on a surface in their original state and then studying their gradual transformation to polymers to GNRs by scanning tunneling microscopy (STM) upon annealing. We performed STM characterization of the precursors of chevron GNRs and 7-AGNRs, and demonstrate that the assemblies of the intermediates of the GNR synthesis are stabilized by π-π interactions. This conclusion was supported by the density functional theory calculations. The resulting monolayer films of GNRs have sufficient coverage and density of nanoribbons for ex situ characterization by spectroscopic methods, such as Raman spectroscopy, and may prove useful for the future GNR device studies.

Laterally extended atomically precise graphene nanoribbons with improved electrical conductivity for efficient gas sensing

Narrow atomically precise graphene nanoribbons hold great promise for electronic and optoelectronic applications, but the previously demonstrated nanoribbon-based devices typically suffer from low currents and mobilities. In this study, we explored the idea of lateral extension of graphene nanoribbons for improving their electrical conductivity. We started with a conventional chevron graphene nanoribbon, and designed its laterally extended variant. We synthesized these new graphene nanoribbons in solution and found that the lateral extension results in decrease of their electronic bandgap and improvement in the electrical conductivity of nanoribbon-based thin films. These films were employed in gas sensors and an electronic nose system, which showed improved responsivities to low molecular weight alcohols compared to similar sensors based on benchmark graphitic materials, such as graphene and reduced graphene oxide, and a reliable analyte recognition. This study shows the methodology for designing new atomically precise graphene nanoribbons with improved properties, their bottom-up synthesis, characterization, processing and implementation in electronic devices.

Atomically precise graphene nanoribbons are a promising platform for tailored electron transport, yet they suffer from low conductivity. Here, the authors devise a strategy to laterally extend conventional chevron nanoribbons, thus achieving increased electrical conductivity and improved chemical sensing capabilities.

Solution Synthesis of Atomically Precise Graphene Nanoribbons

Bottom-up fabrication of narrow strips of graphene, also known as graphene nanoribbons or GNRs, is an attractive way to open a bandgap in semimetallic graphene. In this chapter, we review recent progress in solution-based synthesis of GNRs with atomically precise structures. We discuss a variety of atomically precise GNRs and highlight theoretical and practical aspects of their structural design and solution synthesis. These GNRs are typically synthesized through a polymerization of rationally designed molecular precursors followed by a planarization through a cyclodehydrogenation reaction. We discuss various synthetic techniques for polymerization and planarization steps, possible approaches for chemical modification of GNRs, and compare the properties of GNRs that could be achieved by different synthetic methods. We also discuss the importance of the rational design of molecular precursors to avoid isomerization during the synthesis and achieve GNRs that have only one possible structure. Significant attention in this chapter is paid to the methods of material characterization of solution-synthesized GNRs. The chapter is concluded with the discussion of the most significant challenges in the field and the future outlook.

Interfacial Self-Assembly of Atomically Precise Graphene Nanoribbons into Uniform Thin Films for Electronics Applications

Because of their intriguing electronic and optical properties, atomically precise graphene nanoribbons (GNRs) are considered to be promising materials for electronics and photovoltaics. However, significant aggregation and low solubility of GNRs in conventional solvents result in their poor processability for materials characterization and device studies. In this paper, we demonstrate a new fabrication approach for large-scale uniform thin films of nonfunctionalized atomically precise chevron-type GNRs. The method is based on (1) the exceptional solubility of graphitic materials in chlorosulfonic acid and (2) the original interfacial self-assembly approach by which uniform films that are single-GNR (∼2 nm) thick can be routinely prepared. These films can be transferred to various substrates including Si/SiO₂ and used for the streamlined fabrication of arrays of GNR-based devices. The described self-assembly approach should be applicable to other types of solution-synthesized atomically precise GNRs as well as large polyaromatic hydrocarbon (PAH) molecules and therefore should facilitate and streamline their device characterization.

Aggregation of atomically precise graphene nanoribbons

Atomically precise chevron graphene nanoribbons can form bulk π–π stacked aggregates as well as few-μm-long one-dimensional structures on surfaces that could be used for electronic device fabrication.

Nitrogen-Doping Induced Self-Assembly of Graphene Nanoribbon-Based Two-Dimensional and Three-Dimensional Metamaterials

Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties.

Few-layered titanium trisulfide (TiS3) field-effect transistors

Titanium trisulfide (TiS3) is a promising layered semiconductor material. Several-mm-long TiS3 whiskers can be conveniently grown by the direct reaction of titanium and sulfur. In this study, we exfoliated these whiskers using the adhesive tape approach and fabricated few-layered TiS3 field-effect transistors (FETs). The TiS3 FETs showed an n-type electronic transport with room-temperature field-effect mobilities of 18-24 cm(2) V(-1) s(-1) and ON/OFF ratios up to 300. We demonstrate that TiS3 is compatible with the conventional atomic layer deposition (ALD) procedure for Al2O3. ALD of alumina on TiS3 FETs resulted in mobility increase up to 43 cm(2) V(-1) s(-1), ON/OFF ratios up to 7000, and much improved subthreshold swing characteristics. This study shows that TiS3 is a competitive electronic material in the family of two-dimensional (2D) transition metal chalcogenides and can be considered for emerging device applications.

Bulk properties of solution-synthesized chevron-like graphene nanoribbons

Graphene nanoribbons (GNRs) have received a great deal of attention due to their promise for electronic and optoelectronic applications. Several recent studies have focused on the synthesis of GNRs by the bottom-up approaches that could yield very narrow GNRs with atomically precise edges. One type of GNRs that has received a considerable attention is the chevron-like GNR with a very distinct periodic structure. Surface-assisted and solution-based synthetic approaches for the chevron-like GNRs have been developed, but their electronic properties have not been reported yet. In this work, we synthesized chevron-like GNRs in bulk by a solution-based method, characterized them by a number of spectroscopic techniques and measured their bulk conductivity. We demonstrate that solution-synthesized chevron-like GNRs are electrically conductive in bulk, which makes them a potentially promising material for applications in organic electronics and photovoltaics.

Graphene substrate for inducing neurite outgrowth

A few recent studies demonstrated that graphene may have cytocompatibility with several cell types. However, when assessing cell behavior on graphene, there has been no precise control over the quality of graphene, number of graphene layers, and substrate surface coverage by graphene. In this study, using well-controlled monolayer graphene film substrates we tested the cytocompatibility of graphene for human neuroblastoma (SH-SY5Y) cell culture. A large-scale monolayer graphene film grown on Cu foils by chemical vapor deposition (CVD) could be successfully transferred onto glass substrates by wet transfer technique. We observed that graphene substrate could induce enhanced neurite outgrowth, both in neurite length and number, compared with control glass substrate. Interestingly, the positive stimulatory effect by graphene was achieved even in the absence of soluble neurogenic factor, retinoic acid (RA). Key genes relevant to cell neurogenesis, e.g., neurofilament light chain (NFL), were also upregulated on graphene. Inhibitor studies suggested that the graphene stimulation of cellular neurogenesis may be achieved through focal adhesion kinase (FAK) and p38 mitogen-activated protein kinase (MAPK) cascades. Our data indicate that graphene may be exploited as a platform for neural regenerative medicine, and the suggested molecular mechanism may provide an insight into the graphene control of neural cells.

Oxidative peeling of carbon black nanoparticles

We demonstrate that layered carbon black nanoparticles can be oxidatively peeledviathe reaction with potassium permanganate in sulfuric acid.

Bottom-up solution synthesis of narrow nitrogen-doped graphene nanoribbons

Large quantities of nitrogen-doped graphene nanoribbons can be synthesized via Yamamoto coupling of molecular precursors followed by cyclodehydrogenation using Scholl reaction.